The satellite television reception (TVRO) industry has mushroomed in recent years, and continues to mushroom as more and more people are learning of the vast array of television programming which is accessible to them with the installation of a TVRO earth station. Still other factors are the increasing number of products available to a consumer, and the steadily decreasing cost for such a system. A typical system includes as one of its main components an antenna which is used to collect the signals from the various satellites. As is well known in the art, there are a band of satellites in geosynchronous orbit above the equator which broadcasts television signals, and the antenna's job is to collect those signals from the specific satellite to which it is pointed. At the present time, most of these satellite television signals are broadcast in C-band, or at a frequency range of 3.7 to 4.2 GHz. However, some of the signals are being broadcast at Ku-band, or frequencies ranging from 11.7 to 12.2 GHz. Because of the many advantages offered by Ku-band, more and more programmers are switching to Ku-band, and satellites being placed in orbit are in ever-increasing numbers utilizing Ku-band. Some observers even predict that Ku-band will replace C-band entirely as the C-band satellites end their useful life and fall out of orbit and are replaced by Ku-band satellites.
To take full advantage of the programming available from the satellites presently in orbit, there is a real need for the antenna to be capable of receiving signals at both C-band and Ku-band. To complicate matters further, the signals broadcast at each band are of both horizontal and vertical polarity, so the feed should be capable of receiving and making available for selection both polarizations. Presently, with the C-band feeds well known in the industry, a single polarization rotation device is usually mounted in the feed, and it includes a probe or other signal pick-up structure which can be oriented to select either vertical or horizontal polarity. This capability is desirable to quickly change from one signal to another and thereby view the full complement of television signals broadcast by any one particular satellite. This device also makes correction for skew very easy by slightly moving the probe. However, this device requires that signals of both polarization are present in the same exit port.
The inventors herein are aware of at least two prior art dual frequency feed horns which are shown in U.S. Pat. No. 3,389,394 and U.S. Pat. No. 3,500,419. The '394, patent discloses a multiple frequency feed horn which utilizes a common input of a circular wave guide, the walls of which are used to conduct the low frequency signal to a pair of dipole antennas, and which contains a co-axially mounted dielectric horn which is used to receive the high frequency portion of the signal. This structure has a single feed for the high frequency signal, but utilizes two separate dipole antennas and two separate co-axial connectors and lines to receive the low frequency signal. Similarly, the '419 patent discloses a feed with a high frequency probe extending concentrically through the interior of a low frequency horn, but the horn has four slot apertures for low frequency signals, a pair of apertures being used for each of the two differently polarized signals. A pair of half height wave guides are attached to each pair of slot apertures and are joined in a Y configuration to provide a separate feed for each of the two polarized low frequency signals. Therefore, for the feeds of either of these prior art structures, the polarization rotation device which is presently widely used cannot be utilized, and instead separate low frequency signal pick ups would be required to pick up the two differently polarized signals broadcast at low frequency. Still another more serious problem with these two prior art feeds is that there is no easy way to adjust for skew. With the polarization rotation devices presently available, skew can be easily adjusted by merely rotating the signal pick-up structure. Instead, with the construction of these prior art feeds, the entire feed would have to be rotated.
To solve these and other problems, the inventors herein have succeeded in developing a dual frequency feed which includes a high frequency probe concentrically mounted within a low frequency feed horn, which is highly desirable as it eliminates the problems and complications with offset feeds, and which also incorporates a wave guide attached to the throat of the low frequency feed for conducting low frequency signals of both polarizations such that a polarization rotation device presently available can be mounted to the wave guide and used to select between low frequency signals of different polarizations. In a first embodiment, the wave guide achieves this by utilizing a first turnstile junction mounted adjacent the throat of the low frequency feed which branches into four substantially rectangular, off axis wave guides extending parallel to the central axis of the feed. These wave guides and the low frequency signals conducted through them are then recombined in a second turnstile junction which is co-axial with the low frequency feed, high frequency probe, and first turnstile junction, and which exits through a single circular wave guide and a pair of step transitions into a single polarization rotation device. To facilitate the mounting and stability of the high frequency probe, a collar is provided on the first turnstile junction through which the probe is inserted, the diameter of the collar and probe being matched to provide an engagement therebetween to stabilize the probe in its proper orientation.
The wave guide including the two turnstile junctions and the substantially circular input and output sections can be integrally formed by a plurality of cast aluminum pieces, with flanges formed along the edges of the cast aluminum pieces to facilitate bolting of the pieces together around the high frequency probe. A tuning element may be provided consisting of an upstanding rod axially located in the second turnstile junction to reject the unwanted low frequency modes and direct the waves into the exit guide. The step transitions at the exit portion of the guide permit the higher order modes to die out before reception by the probe of the low frequency polarization rotation device. Also, a mode ring is fitted to the mouth of the throat of the wave guide to improve the illumination pattern of the feed, as is well known in the art.
Still another feature of the present invention is the construction of the high frequency probe. Generally, the high frequency probe may be a hollow metal cylinder, such as aluminum. However, to adapt the high frequency probe for use with the same reflector as is utilized for the low frequency band, a dielectric plug is utilized to "spoil" the Ku-band beam and thereby increase the electrical aperture of the probe. This broader beam width substantially de=sensitizes the placement of the Ku-band probe, and helps to minimize the effect on performance from improper installation, or shifting of the position of the feed over time due to weathering, wind loading, or the like. This dielectric insert may be a cast polystyrene plug which is simply inserted within the tip of the probe.
As mentioned above, the feed of the present invention permits reception of both C-band and Ku-band signals through a single feed where the signals are co-mingled at the horn input, and where the low frequency signals of both polarization are propagated through a single wave guide to a single exit port where the low frequency signal of either polarization may be detected or picked up with the presently known polarization rotation device. This is achieved with a Ku-band probe and C-band feed which are co-axially aligned for optimum utilization of the reflector and antenna.
In a second embodiment of the present invention, the off-axis rectangular wave guides may be eliminated and replaced by co-axial cables with probes extending into the square portion of circular-to-square transitions, thereby forming cable turnstile junctions, mounted both at the throat of the feed and at the transition to the low frequency polarization rotation device. These co-axial cables have probes for receiving the signal within the cable turnstile and launching the signal at the other end. Care must be taken to maintain the length of the co-axial cables so that there is no phase imbalance or power mismatch at the output cable turnstile. However, if manufactured properly, this embodiment does provide some cost savings over the cast aluminum off-axis rectangular wave guides of the first embodiment.
In a third embodiment of the present invention, the co-axial cables are utilized, but their associated probes are inserted through the outer mode ring of the feed, and not into a cable turnstile junction connected to the throat. With the Ku-band probe inserted through the inner throat of the feed, the inner throat acts as a reciprocal dummy to excite the proper mode within the mode ring, as desired. Thus, the high frequency probe receives and detects the high frequency signal, while the four low frequency probes mounted to the outer ring receive the low frequency signal. As C-band transmission is in both vertical and horizontal polarization, the four low frequency probes are best positioned symmetrically about the circular mode ring, with the top and bottom probes thus receiving vertically polarized signals, and the right and left probes receiving horizontally polarized signals. These separately detected signals are then re-combined in a cable turnstile junction within which a second set of probes are mounted at the other ends of the co-axial cables. This embodiment may not achieve the same gain as is thought to be attainable in some of the other embodiments of the present invention, but it does benefit from a further anticipated cost reduction by eliminating the first cable turnstile as is used in the second embodiment of this invention.
In a fourth embodiment of the invention, an orthomode junction (which is essentially a turnstile junction having two of its outputs shorted) is connected through a circular-to-square transition to the throat of the feed, and the Ku-band probe band is inserted through the back of the orthomode junction and concentrically within the throat of the feed as in the other embodiments. This embodiment does provide co-mingling of both high frequency and low frequency signals at the throat of the feed, but requires two separate low frequency pick-up means at its output to detect and receive both polarizations of the low frequency signal. Thus, this embodiment does not provide the inherent advantage offered by the other embodiments of this invention in that two low frequency signal pick-up means must be used, but it does offer a simpler design and anticipated lower cost to construct than some of the other embodiments. Furthermore, this embodiment also requires rotation of the feed to adjust for skew, although its simpler construction, and anticipated lighter weight does alleviate this problem somewhat. In a broad sense, the orthomode junction which is used to terminate the wave guide, is in the same family as the turnstile junctions utilized in the other embodiments. Hence, when the term "turnstile junction" is used herein, it is meant to refer to any of these constructions.
In the foregoing description and explanation of the present invention, it has been assumed that its major application has been to the TVRO industry, and, in particular, as a feed means with an antenna having a main reflector. However, this need not necessarily be the case as the feed itself can and does function as an antenna for low gain applications. This can include applications wherein data is transmitted through spread spectrum technology. Furthermore, the frequencies mentioned herein are C-band and Ku-band. However, it is anticipated that these bands may themselves be replaced in coming years such that still higher frequency bands are utilized thereby making the feed of the present invention more suitable for direct use as an antenna by itself. Thus, the inventors herein anticipate that this invention has applications well beyond the specific embodiments and applications disclosed herein.
The foregoing has been a brief description of some of the principal advantages and features of the present invention which may be more fully understood by referring to the drawings and description of the preferred embodiment which follows.